Modern high-efficiency combustion turbines have firing temperatures that exceed about 2000° F. (1093° C.), and firing temperatures continue to increase as demand for more efficient engines continues. Gas turbine components, such as nozzles and blades, are subjected to intense heat and external pressures in the hot gas path. These rigorous operating conditions are exacerbated by advances in the technology, which may include both increased operating temperatures and greater hot gas path pressures. As a result, components, such as nozzles and blades, are sometimes cooled by flowing a fluid through a manifold inserted into the core of the nozzle or blade, which exits the manifold through impingement holes into a post-impingement cavity, and which then exits the post-impingement cavity through apertures in the exterior wall of the nozzle or blade, in some cases forming a film layer of the fluid on the exterior of the nozzle or blade.
The cooling of the trailing edge of a turbine airfoil is important to prolong its integrity in the hot furnace-like environment. While turbine airfoils are often made primarily of a nickel-based or a cobalt-based superalloy, turbine airfoils may alternatively have an outer portion made of one or more ceramic matrix composite (CMC) materials. CMC materials are generally better at handling higher temperatures than metals. Certain CMC materials include compositions having a ceramic matrix reinforced with coated fibers. The composition provides strong, lightweight, and heat-resistant materials with possible applications in a variety of different systems. The materials from which turbine components, such as nozzles or blades, are formed, combined with the particular conformations which the turbine components include, lead to certain inhibitions in the cooling efficacy of the cooling fluid systems. Maintaining a substantially uniform temperature of a turbine airfoil maximizes the useful life of the airfoil.
The manufacture of a CMC part typically includes laying up pre-impregnated composite fibers having a matrix material already present (prepreg) to form the geometry of the part (pre-form), autoclaving and burning out the pre-form, infiltrating the burned-out pre-form with the melting matrix material, and any machining or further treatments of the pre-form. Infiltrating the pre-form may include depositing the ceramic matrix out of a gas mixture, pyrolyzing a pre-ceramic polymer, chemically reacting elements, sintering, generally in the temperature range of 925 to 1650° C. (1700 to 3000° F.), or electrophoretically depositing a ceramic powder. With respect to turbine airfoils, the CMC may be located over a metal spar to form only the outer surface of the airfoil.
Examples of CMC materials include, but are not limited to, carbon-fiber-reinforced carbon (C/C), carbon-fiber-reinforced silicon carbide (C/SiC), silicon-carbide-fiber-reinforced silicon carbide (SiC/SiC), alumina-fiber-reinforced alumina (Al2O3/Al2O3), or combinations thereof. The CMC may have increased elongation, fracture toughness, thermal shock, dynamic load capability, and anisotropic properties as compared to a monolithic ceramic structure.